In geophysics, electric and electromagnetic (EM) methods are used to measure the electric properties of geologic formations. At low frequencies, rock resistivity accounts for almost all of the electromagnetic response. Because replacement of saline pore fluids by hydrocarbons increases the resistivity of reservoir rocks, EM methods are important exploration tools. Seismic methods had traditionally been used for detection of such geologic formations, however, the results could be ambiguous.
Several electromagnetic methods have been developed for mapping sub-seafloor resistivity variations. See, for example, U.S. Pat. No. 5,770,945 of Constable (magnetotelluric (MT) methods), U.S. Pat. No. 7,116,108 of Constable (MT and controlled electromagnetic (EM) source methods), U.S. Pat. No. 7,109,717 of Constable (controlled EM source for monitoring), U.S. Pat. No. 6,522,146 OF Srnka (controlled EM source), International Publication No. WO 03/048812 of MacGregor and Sinha (controlled EM source), and International Publication No. WO 01/57555 of Rees (controlled EM source). The disclosure of each of the identified patent documents is incorporated herein by reference.
The magnetotelluric (MT) method is an established technique that uses measurements of naturally occurring electromagnetic fields to determine the electrical resistivity, or conductivity, of subsurface rocks. An MT survey employs time series measurements of orthogonal components of the electric and magnetic fields, which define a surface impedance. This impedance, observed over a broad band of frequencies and over the surface, determines the electrical conductivity distribution beneath that surface, with horizontal layers of the earth being mathematically analogous to segments of a transmission line. Principal factors affecting the resistivity of subsurface materials include temperature, pressure, saturation with fluids, structure, texture, composition and electrochemical parameters. Resistivity information may be used to map major stratigraphic units, determine relative porosity or support a geological interpretation. A significant application of MT surveying is oil exploration. An MT survey may be performed in addition to seismic, gravity and magnetic data surveys. A combination of data from two or more different survey methods leads to a more complete understanding of subsurface structure than may be possible through the use of any single technique alone, particularly where the structure is such that measurement using a given technique may be contraindicated.
For example, certain structures such as sediments buried under salt, basalt or carbonate have poor seismic performance and productivity. These structures generate strong reflections and reverberations, making imaging of the buried sediments difficult using acoustic methods alone. On the other hand, because the MT method does not involve the measurement of responses to artificially-created seismic events, it can be utilized in lieu of, or in combination with, seismic methods to minimize the error induced by reflections.
Electric fields are well suited to applications in seawater. Transmitter currents can be passed through seawater with simple electrode systems and relatively low power consumption by way of towed transmitter antennas towed through the seawater. Controlled source electromagnetic (CSEM) methods have been shown to be useful in evaluation of reservoir resistivity for targets in very shallow water to as deep as several thousand meters, achieving seafloor penetration depths as great as 5-10 km in 5 km of water. Such methods involve deployment of a seafloor receiver with orthogonal antennae and towing an electric field transmitter near the seafloor at some distance away. The transmitter is towed close to the seafloor to maximize the coupling of electric and magnetic field with seafloor rocks. Such methods have provided significant economic savings in terms of avoiding the costs of drilling test wells into sub-seafloor structures that do not contain economically recoverable amounts of hydrocarbon. In addition, these methods could be used for positioning wells for optimal recovery based on the shape of the reservoir.
The current technologies described above typically require antennas on the order of 10 m to make low noise measurements of seafloor electric fields. It is known that the use of a long antenna, known as a “LEM”, on the seafloor (100 m long or more) provides a better signal to noise ratio for electric field measurements than existing instruments. An exemplary LEM deployment is shown in FIG. 1a. (See, e.g., A. D. Chave, et al., “Electrical Exploration Methods for the Seafloor” in Electromagnetic Methods in Applied Geophysics, Vol. 2, M. Nabighian (ed.), Soc. Explor. Geophys., 1991.)
Although great progress has been made by industry in the past few years, the collection of seafloor data is still a technologically sophisticated exercise, and further development of the technology is needed. Currently, the bulk of marine CSEM surveys are carried out to assess the resistivity of targets previously identified by seismic methods, often based on structure but usually including a seismic DHI (direct hydrocarbon indicator). The next frontier in the use of marine CSEM is to extend the depth capability to identify the resistivity of structural targets that are too deep to exhibit a seismic DHI. Although the physics of diffusive EM propagation will always require that the target size be significant compared to the depth of burial, currently the noise floor of the transmitter/receiver system, along with the quality of transmitter waveform stability and navigation, are limiting factors in detecting deep resistivity targets.
Accordingly, the need remains for improvements to instrumentation systems and methods to enhance signal quality and sensitivity in CSEM surveys. The present invention is directed to such improvements.